PCM Modem Standard: ITU draft recommendation V.90

Joe Decuir, Microsoft, Windows Operating Systems Division

ITU V.90 presentation, outline

• Phone lines and their limits

• How traditional modems work, up to V.34

• How PCM modems get so fast

• Why PCM modems aren’t faster.

• Some details of V.90

• Where we go from here

Summary of phone system

• The 2-wire pair to your house is analog.

• The analog pair is DC feed plus duplex AC.

• AC includes 20Hz 86Vac ringing and loads

• The inside is digital, limited by the codecs: – 4kHz bandwidth, useful from 200-3700 Hz– 8kHz sample rate – 8 bit companded A/D/A (ITU G.711)

G.711 PCM coding

• 8 bits/value: 1 sign, 3 exponent, 4 mantissa

• 16 segments, each with 16 linear steps

• Step size at full-scale is 128 times step size at origin.

• Monotonic, but not linear

• 13 bit total dynamic range

• Two variations: mu-Law, A-Law

G.711 Transfer curve

Analog values

codes

High precision near the origin

Low precision at high levels.

Network Impairments

• Tandem Encoding: digital-analog-digital..

• ADPCM encoding (G.721, G.726)

• Loaded loop

• Robbed Bit Signaling (RBS)

• Digital Loss Pads

• A-Law/mu-Law conversion

More Network Impairments

• D/A converter non-linearity's

• Frequency dependent non-linearity

• Talker echo

• Residual loop noise

• Residual loop distortion

How modems work

• Establish channels by some means:– frequency division multiplex (FDM)– time division multiplex (TDM)– echo cancellation multiplex (ECM)

• Map data into symbols: – Frequency shift key (FSK) (1 bit/symbol)– Phase shift key (PSK) (1-3 bits/symbol)– Quadrature amplitude modulation (QAM)

• Bit rate = symbol rate x bits/symbol

Legacy analog modems

• Bell 103 (1960s): FDM, FSK, 0-300 bit/s

• Bell 212 (1970s): FDM, PSK, 1200 bit/s

• V.22bis (1984): FDM, QAM, 2400 bit/s

• V.32 (1986): ECM, QAM, 9600 bit/s

• V.32bis (1991): ECM, QAM, 14400 bit/s

• V.34 (1994): ECM, QAM, 28800 bit/s

• V.34 (1996): ECM, QAM, 33600 bit/s

How V.34 got so fast, 1 of 3

• Probes the line at startup for maximum bandwidth (V.32 uses 2400 Hz symbol rate; V.34 max is 3429, 10/7ths larger)

• Probes the line attenuation characteristics, and pre-emphasizes (like Dolby).

• Advanced forward error control, 16 state 4D Wei codes, 20% overhead.

How V.34 got so fast, 2 of 3

• Adaptively adjusting equalization and precoding.

• Nonlinear Encoding: introduce distortion to compensate for PCM encoding.

• Shell Mapping: map data bits to signal points in a multidimensional constellation, partitioning a 2-D signal constellation into rings containing an equal number of points.

How V.34 got so fast, 3 of 3

• Given all the improved S/N ratio from the preceding tricks, using very dense symbol constellations:– V.32 (1986): 32 points - 4+1 bits/symbol– V.32bis (1991): 128 points - 6+1 bits/symbol– V.34 (1994): 960 points - net 8.4 bits/symbol– V.34 (1996): 1664 points - net 9.8 bits/symbol

3

31

1

-1-1-3

-3

001 110 101 010

100 000011 111

101 010 001 110

000 111 100 011

(Re)

(Im )

0 5 16 32 56 8 5 12 2 16 3 21 3

4 8 21 38 63 93 1 27 171 2 19

2 7 18 36 59 88 124 166 2 17

15 2 0 3 0 4 9 72 101 138 182 230

31 37 48 65 91 118 155 198

54 62 71 90 112 141 180 221

83 92 100 117 140 172 208

121 125 137 154 179 207

162 170 181 197 220

212 218 228236 224 216

234 206 185 173 164

226 193 165 146 133 123

229 189 156 131 110 96 87

201 160 126 98 79 64 58

222 177 135 102 77 55 41 35

203 158 119 84 60 39 24 17

194 148 108 75 50 28 13 6

186 142 103 69 43 22 9 1238

190 144 106 73 45 25 11 3

199 152 113 80 52 33 19 12 10 14 26 42 66 97 134 174 225

210 167 128 94 67 47 34 27 23 29 5740 81 111 147 187 237

232 183 149 115 89 68 53 46 44 51 61 78 99 132 168 209

214 175 139 116 95 82 74 70 76 86 104 129 157 195 235

205 176 150 130 114 107 105 109 120 136 161 191 227

231202178159151143145153169184215

233 211 200 192 188 196 204 223

239

1

-3

-7

-11

-15

-19

-23

-27

-31

-35

5

9

1 3

17

21

25

29

33

37

1

-3

-7

-11

-15

-19

-23

-27

-31

-35

5

9

13

17

21

25

29

33

37

1-3-7-11-15-19-23-31-35-39 5 9 13 17 21 25 29 33 37-27

1-3-7-11-15-19-23-31-35-39 5 9 13 17 21 25 29 3 3 37-27

(Re)

(Im)

How can you go faster?

• Change the system: go digital on one side, with ISDN or T1 lines:– Only one analog loop - halve the impairments.– No echo from digital side.– No quantization noise in downlink direction.– Direct access to the G.711 PCM clock: 8kHz– Direct access to the G.711 PCM DAC: 8 bits

Traditional Modem Model

Local Loop

PCM Encoder

PCM Decoder

Line Card

PCM DecoderDIGITAL

BACKBONENETWORK

Line Card

PCM Encoder

Local Loop

H H

Analog Modem

Analog Modem

PCM Digital Modem Model

PCM DecoderDIGITAL

BACKBONE

NETWORK

Line Card

PCM Encoder

Local Loop

H

Downstream

Upstream

Digital Modem

Analog Modem

Why can’t we go 64000 bit/s?

• Residual noise hurts close to the origin.• Signal power limits clip peak code values.• Codec filters hurt close to 4kHz.• Line card transformers won’t pass DC.• Ring detectors are a big load below 100Hz.• Other digital impairments, particularly RBS

and Digital Loss Pads.

Bit rate limiting envelope

frequency

Signal

power

noise floor, from codec quantizer

max signal,

from FCC

minimum frequency,

from DC and Ring detectors

maximum frequency, from codec filters

-10dbm

-48dbm

3900 Hz70Hz

How do we get close to 56K?

• Adaptively probe the line, and determine the operational limits

• Mapping and framing: multiple modulus conversion (3Com) - fractional bits/symbol

• Convolutional Spectral Shaping (Motorola)

• Digital Impairment Detection & Mitigation.

b0

b1

bK-1dD-1

d0

BitParser

ScrambledDataBits

ModulusEncoder

M0

M1

M2

M3

M4

M5

K5

K4

K3

K2

K1

K0

M5-pointMap

M4-pointMap

M3-pointMap

M2-pointMap

M1-pointMap

M0-pointMap

U5

U4

U3

U2

U1

U0

PCM5

PCM4

PCM3

PCM2

PCM1

PCM0

MuxSerialPCMOctets

SpectralShapers0

sS-1

Sr

SignsAssign

$0 $5

K, bits enteringmodulus encoder

S, sign bits used foruser data

Data Signalling Rate,kbit/s

From: To: From: To:15 6 6 28 2816 5 6 28 29 1/317 4 6 28 30 2/318 3 6 28 3219 3 6 29 1/3 33 1/320 3 6 30 2/3 34 2/321 3 6 32 3622 3 6 33 1/3 37 1/323 3 6 34 2/3 38 2/324 3 6 36 4025 3 6 37 1/3 41 1/326 3 6 38 2/3 42 2/327 3 6 40 4428 3 6 41 1/3 45 1/329 3 6 42 2/3 46 2/330 3 6 44 4831 3 6 45 1/3 49 1/332 3 6 46 2/3 50 2/333 3 6 48 5234 3 6 49 1/3 53 1/335 3 6 50 2/3 54 2/336 3 6 52 5637 3 5 53 1/3 5638 3 4 54 2/3 5639 3 3 56 56

Digital Impairment Training

• The analog modem sends a “DIL Descriptor” (Table 12/V.90) to the digital modem, which is used to generate a PCM signal sequence back to the analog modem.

• The analog modem continues this process until it has identified the digital network impairments, and informed the digital modem of how to compensate.

Example DIL Parameters

• N, number of segments (0-255)

• Lsp, Ltp, lengths of sign & training patterns

• SP, Single Sign Pattern bit map sequence.

• TP, Training Pattern bit map sequence.

• H1-8 signal length multipliers (L=6*(H+1))

• REF1-8 reference symbol values.

Uses for DIL sequences• Identify and locate RBS

• Calibrate digital loss pads

• Calibrate the PCM codecs

• Recognize and compensate for A-Law to mu-law conversion.

What do we get realistically?

• In Microsoft’s WHQL test lab, with average line simulators, X2™ units typically ran at 50K, K56flex™ units typically ran 44K.

• Modem vendors contributed new tricks, hoping to bargain later on patents. V.90 products benefit, but the lawyers do, too.

• WHQL is tooling up to do V.90 inter-operability, but we don’t have new data yet.

Is this asymmetric?

• X2, K56Flex and V.90 are asymmetric, with a V.34 uplink and a PCM downlink.

• Lucent has proposed means to calibrate the line delay from the user modem to the local office.

• This trick will allow a PCM uplink as well, maybe 44K bit/s or higher.

• That may make IP telephony & H.323 video-conferencing usable on modems.

What does this cost?

• Not much, actually!• There is a lot less DSP work to do per

symbol for PCM modems vs V.34.• But, there are more symbols: 8000 vs 3429• The analog front ends are similar.• Result: the hardware is essentially the same.

Private estimate are 20MIPS for both.• There are at least 8 vendors planning to

offer software based modems.

The V.90 standards process

• Nobody wants to buy a modem that is obsolete in less than a year.

• ITU Study Group 16, fed by TIA TR-30.1, drove this aggressively, meeting monthly.

• They “determined” a draft in February.

• They plan to “decide” it next September.

• Anything that says “V.90” today is ‘beta’.

Where to go from here?

• The second version of V.90 (“Issue 2”) will address duplex PCM modulation.

• All the modem vendors are working on ITU “G.lite”, from Q4/15, also known as Universal Asymmetric Digital Subscriber Line (UADSL). It uses the same phone wire, but with much wider spectrum, and runs a lot faster.

References (1 of 2)

• Modems: – Theory and Practice of Modem Design,

Bingham, Wiley Interscience.– Data Communication, Lee & Messerschmidt,

KAP, book on modem DSP

• V.34 and V.90:– The V.34 High Speed Modem Standard– The Information Driveway– IEEE Communications Magazine, vol34.12

References (2 of 2)

• ITU-T V.34-1996• ITU-T Q.23/16 TD70(Plen), Proposed text

for V.pcm, February 4, 1998.